The neutron-induced fission cross section of Th-232 has been measured from 1 eV to 20 keV using a lead slowing time spectrometer. The cross section was normalized to the U-238(n,f) subthreshold cross section. In the energy region between 1 and 500 eV the fission cross section has a 1/upsilon shape, and has a small peak near 5 keV. By extrapolation of 1/upsilon the thermal fission cross section of 54 +/- 6-mu-b was deduced. The observed 1/upsilon and small-peaked cross sections were interpreted as the fission through the vibrational states in the second well of a triple-humped fission barrier of Th-233.

The 238U(n,f) cross section has been measured from 3 eV to ≃ 100 keV with the Rensselaer Intense Neutron Spectrometer, a 75-ton lead slowing down spectrometer at the Gaerttner Laboratory at Rensselaer Polytechnic Institute. Four fission ionization chambers containing a total of ≃ 0.8 g of 238U (4.1 ppm 235U) were used for the measurements. The fission widths of the 6.67-, 20.9-, and 36.8-eV resonances were measured as (10 ± 1), (58 ± 9), and (12 ±2) neV, respectively. By combining these fission results and published resonance parameters, the 238U thermal fission cross-section contribution from positive energy resonances was determined to be (2.7 ± 0.3) µb. The resonance fission integral from 0.4 eV to 100 keV was determined to be (1.30 ±0.15) mb.

Subthreshold fission is observed in $^{238}\mathrm{U}(n, f)$ at 0.720, 1.210, 2.5, 7.5, 11, 15, 27, and 35 keV. Fission widths are obtained for the resonances at 720 and 1210 eV. The average fission cross section is 87 \ifmmode\pm\else\textpm\fi{} 26 \ensuremath{\mu}b for $10<~{E}_{n}<~30$ keV and 40 \ifmmode\pm\else\textpm\fi{} 12 \ensuremath{\mu}b for $30<~{E}_{n}<~100$ keV. The second potential minimum is deduced to lie \ensuremath{\approx} 2.2 MeV above the first minimum.